You shouldn't have to worry about the contents of your hard drive. I would first ask them, though, if your hard drive has been/is going to be affected. If so, I would ask them to remove it, put it in one of their machines, back it up, and then go on with the procedure. Unless, that is, if it is your HD causing the troubles. My guess is that this is not the case. I think I have heard that the vid card in the new iMacs acts up in some cases. Your HD should be ok.